extracellular DNA

The first thing that comes up in a Google search for “Hoechst” is the family of fluorescent dyes used to stain DNA in cells before microscopy. The Hoechst dyes derive their names from their manufacturer: a company, now part of Sanofi, named after the town where it was founded, which is now part of Frankfurt, Germany. The word itself means â€œhighest [spot].â€

Although DNA runs the show in every cell, itâ€™s usually well-hidden inside the nucleus or the mitochondria. Extracellular DNAâ€™s presence is a signal that injury is happening and cells are dying.

Biomedical engineer Mike Davis and collaborator Niren Murthy have been exploiting the properties of a DNA-binding dye called Hoechst 33342, often used to stain DNA in cells before microscopy. The dye can only bind DNA if it can get to the DNA â€“ that is, if membranes are broken. This property makes the dye a good way to target injured tissue, either as an imaging agent or for therapy.

In addition, in a recent paper published in Scientific Reports, Davis and his colleagues attach the Hoechst dye to the cardioprotective growth factor IGF-1, creating a version of IGF-1 that is targeted to injured heart muscle. The first author of the paper is cardiology fellow Raffay Khan, MD.

IGF-1 has shown a lot of potential for treating heart disease, but itâ€™s not the most cooperative as a drug, because it doesnâ€™t last long in the body and doesnâ€™t stick around in the heart. Linked up to the dye, IGF-1 behaves better. When used to treat mouse hearts after a heart attack, the Hoechst-IGF-1 treated-hearts have better function and less scar tissue (seen here as red).

The authors conclude:

With the broad chemistry surrounding functionalized PEG used to create Hoechst derivatives, it may be possible to target other therapies such as cells, small molecules, and even nanoparticles. We believe that the use of DNA binding agents such as Hoechst can be used to target exposed DNA in other diseases where necrotic cell death plays a critical role and could be used as a platform therapy.

DNA usually occupies a privileged place inside the cell. Although cells in our body die all the time, an orderly process of disassembly (programmed cell death or apoptosis) generally keeps cellular DNA from leaking all over the place. DNA’s presence outside the cell means something is wrong: tissue injury has occurred and cells are undergoing necrosis.

Researchers from the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University have devised a way to exploit the properties of extracellular DNA to create an imaging agent for injured tissue. Niren Murthy and Mike Davis recently published a paper in Organic Letters describing the creation of â€œHoechst-IR.â€ This imaging agent essentially consists of the DNA-binding compound Hoechst 33258 (often used to stain cells before microscopy), attached to a dye that is visible in the near-infrared range. A water-loving polymer chain between the two keeps the new molecule from crossing cell membranes and binding DNA inside the cell.